For treads for high performance tires, generally high level grip performance and abrasion resistance are demanded. So far, in order to obtain a rubber composition exhibiting high grip performance, there have been known a rubber composition prepared using, as a rubber component, a styrene-butadiene copolymer rubber (SBR) having a high glass transition temperature (Tg), a rubber composition prepared by compounding, in a rubber component, a resin having a high softening point as a substitute for a part of process oil in an amount equivalent to that of the substituted process oil, a rubber composition containing a softening agent or a carbon black in a high amount, a rubber composition prepared using a carbon black having a small particle size, or a rubber composition prepared by blending a combination of the above-mentioned SBR, resin having a high softening point, softening agent or carbon black.
However, in the case of a rubber composition prepared using SBR having a high Tg, there is a problem that temperature dependency is increased and variation of performance caused by a temperature change becomes large. Also, in case of a rubber composition where a resin having a high softening point is compounded to a rubber component as a substitute for a part of process oil in an amount equivalent to that of the substituted process oil, there is a problem that temperature dependency increases due to an influence of the resin having a high softening point if the substitution amount is large. Further, in the case of using a carbon black having a small particle size or a softening agent in a large amount, there is a problem that dispersion of the carbon black is not good and abrasion resistance is lowered.
In order to solve these problems, a rubber composition prepared using a low molecular weight styrene-butadiene copolymer has been proposed (See Patent Document 1). However, since the low molecular weight styrene-butadiene copolymer has a double bond having crosslinkability, there is a problem that a part of low molecular weight components form crosslinking with a matrix rubber component to be incorporated in the matrix and hysteresis cannot be inhibited sufficiently. Also, when double bonds are modified to saturated bonds by hydrogenation in order to prevent low molecular weight components from being incorporated in the matrix as a result of crosslinking, there arise a problem that compatibility with the matrix is significantly lowered and as a result, bleeding of the low molecular weight components occurs.
In order to inhibit bleeding of the low molecular weight components, there is a method of increasing a styrene content in the low molecular weight styrene-butadiene copolymer up to 40% or more. However, there is a problem that if the styrene content is increased, hardness increases, which makes handling difficult.
The present situation is such that, in any of the above-mentioned cases, so far there has not been obtained a rubber composition for a tire tread which can solve the above-mentioned problems at a high level.
Myrcene is a natural organic compound and is a kind of olefin belonging to monoterpene. There are two kinds of isomers as myrcene such as α-myrcene (2-methyl-6-methyleneocta-1,7-diene) and β-myrcene (7-methyl-3-methyleneocta-1,6-diene). In Patent Document 2, a polymer of myrcene is disclosed.
Farnesene is a kind of isoprenoid compound synthesized chemically by oligomerization of isoprene or dehydration reaction of nerolidol, and is used mainly as a perfume or its starting material (Patent Document 3).